Breast cancer is the leading site of new cancer cases in women and is the second leading cause (after lung cancer) of cancer death among women. The high mortality rate associated with breast cancer, however, is due to a propensity for these tumors to spread while the primary tumors are small and undetected. The molecular mechanisms underlying the upstream regulation of the Wnt1 and STAT3, two major transforming molecules in human cancer and, in turn, progression and metastasis of these cancers are not completely understood at the present time, but believed to involve perturbation of master chromatin modifiers. For example, overexpression of metastatic tumor antigen 1 (MTA1), a master chromatin modifier, is frequently associated with an aggressive clinical course in human breast cancer. Despite the remarkable growth of information about MTA1, Wnt1, and STAT3, knowledge regarding the mechanism by which MTA1 and the direct targets of MTA1-containing coregulatory complexes regulate mammary epithelial cell transformation and metastasis remains elusive, and is the focus of this application. In this context, our recent work suggests that MTA1 deregulation in mammary epithelial cells plays a significant role in stimulating Wnt1 via targeting of specific gene chromatin, namely repressing Six3 (a direct corepressor of Wnt1), and STAT3 chromatin. In addition, for the first time, we discovered that MTA1 is required for breast-to-lung metastasis in a transgenic mouse model. This proposal is designed to establish the mechanism by which MTA1, a physiologic upstream regulator of Wnt1 and STAT3 gene chromatin, contributes to the development of neoplastic phenotypes in mammary epithelial cells and tumors. These findings offer a unique opportunity to study the first upstream common chromatin modifier of two pathways implicated in oncogenesis. Our working hypothesis is that deregulation of MTA1 results in activation of the Wnt1, and STAT3 pathways, and consequently, confers neoplastic and metastatic properties to breast epithelial cells. To address these hypotheses, our Specific Aims are to: (1) Determine the mechanism of MTA1 regulation of Wnt1 expression, signaling, and functions in mammary epithelial cells; (2) Determine mechanism of MTA1-driven breast-to-lung metastasis; (3) Determine the expression characteristics and significance of MTA1 and its targets/effectors in human breast cancer. An innovative aspect of our proposal is the delineation of the mechanistic and functional significance of the MTA1- Wnt1, and the MTA1-STAT3 pathways in breast cancer cells. These studies will uniquely define the mechanisms of neoplastic and metastatic activities of MTA1. Our proposed research is significant, as the knowledge gained from this research will enhance our understanding of the critical regulatory pathways with established roles in breast cancer progression. In addition, this research will form the basis for new translational advances in identifying novel molecular targets, detecting, and treating breast cancer, by identifying MTA1 as a key master regulatory nodule.